Coming to a new league almost always requires a player to get used to new things, physicallity, speed of play etc, but unfortunately for Sancho, by the time he had got used to EPL football, the team was starting to perform badly.
One of United's biggest problems being the speed at which the whole team would move forward, I don't know how many times during a game we would get a quick break, get the ball to a forward player, then play the ball backwards to a defender. Much of the problem, I believe, was the lack of support that the full backs and midfield gave, moving into attacking positions, more often than not staying deep to receive the ball backwards from the forward player. Sancho, and other wider players, would need players moving beyond them, to keep the ball going towards the opposition goal.
I believe that next season Sancho will thrive, as I hope Rashford, Garnacho and others will do with a new, more attacking system, but probably, and most importantly, we require a strong defence, and at least one midfield player who understands defensive duties.